lecture vs public lecture

lecture and public lecture both are nouns.

Word	Noun	Adjective	Verb	Adverb
lecture	Yes	No	Yes	No
public lecture	Yes	No	No	No

As nouns, lecture and public lecture are synonyms defined as:

Other synonyms of lecture include talk.

lecture (noun)	public lecture (noun)
teaching by giving a discourse on some subject (typically to a class)	a speech that is open to the public
a speech that is open to the public
a lengthy rebuke

lecture (verb)	public lecture (verb)
censure severely or angrily
deliver a lecture or talk